The present invention relates to improvements in the quality of the alarm sound generated by alarm electronic timepieces.
Generally, electronic timepieces display time by dividing outputs of a reference signal oscillator employing a quartz crystal or the like. In an alarm electronic timepiece, it is convenient to use the signal produced from the timepiece circuit portion as a signal source which drives a sound-producing device such as a speaker or the like.
FIG. 1 shows a circuit construction of a conventional timepiece circuit portion in which: an audible signal of 4096 Hz (see FIG. 2a) and an interrupting signal of 8 Hz (see FIG. 2b) are produced from the dividing stages of the timepiece circuit portion (not shown) and are fed to the input terminal of an AND circuit 101. The output from the AND circuit 101 and an alarm signal are fed to an AND circuit 102 and the output from the AND circuit 102 is connected to the base of an activating transistor 72. A speaker 14 is connected to the collector of the transistor 72. By the above mentioned construction, the audible signal of 4096 Hz is modulated by the intermittent signal of 8 Hz to produce the output signal shown in FIG. 2c from the AND circuit 101 and the output signal is fed to the base of the transistor 72 through the AND circuit when the alarm set signal is at high level and activates the speaker 14. The alarm set signal comprises a signal produced when the alarm sound is necessary, for instance at the predetermined alarm time or time service, though such is not shown in the drawing. The wave shape of the sound wave produced from the speaker 14 by the above mentioned construction and operation is, as shown in FIG. 3, the shape of 4096 Hz intermittent by 8 Hz. The advantage of the intermittent alarm sound is that it breaks the monotony of a continuous sound; however, this type conventional intermittent alarm sound is disadvantageous since the sound gives listeners an unpleasant feeling since the intermittence is carried on taking the shape of a square wave which produces an unnatural sound.
Accordingly, it is an object of the present invention to remove the above mentioned unpleasant feeling as much as possible by providing circuitry to obtain a more natural and pleasant sound by means of an easy logical operation within the timepiece.
Another object of the present invention is to improve the alarm sound produced by an alarm electronic timepiece by varying the duty cycle suitably taking advantage of the variation of the sound pressure caused by variation of the duty cycle of the audible signal. The signal referred to as the "audible signal" in this disclosure designates the signal within the audible frequency region which comprises the basic sound wave as opposed to the signal which determines the alarm time or the intermittent signal among the various signals fed to the alarm circuitry.